Proposal to add the 3rd bad use of namespaces in tests, because people feels the level-2 "test" is a good choice:

namespace mymodule;                     // Violates the level1 rules - invalid component name
namespace mod_breakfast\myutilities;    // Violates the level2 rules - invalid core API name
namespace mod_forum\test;               // While technically correct ("test" is a valid API) - it's not allowed
                                        // because namespaces in tests should point to the namespaces being
                                        // covered by the test, and normally components don't have any "test"
                                        // level-2 to be covered. Only then it can be used.